The Ringworld needed light and dark cycles to provide the inhabitants with day and night. So, shadow squares were placed in orbit closer to the sun; the squares were connected to each other with a very fine wire.

The makers of Ringworld never intended that the inhabitants would ever encounter shadow square wire. But there it was.

"Don't touch it," Louis called... a girl had lost some fingers trying to pick up shadow square wire.

Close up, it still looked like black smoke... You could see the black thread, if your eye was within an inch of it; but then your eye would water and the thread would disappear. The thread was that close to being invisibly thin.